In2S3 Quantum Dots: Preparation, Properties and Optoelectronic Application
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Low-dimensional semiconductors exhibit remarkable performances in many device applications because of their unique physical, electrical, and optical properties. In this paper, we report a novel and facile method to synthesize In2S3 quantum dots (QDs) at atmospheric pressure and room temperature conditions. This involves the reaction of sodium sulfide with indium chloride and using sodium dodecyl sulfate (SDS) as a surfactant to produce In2S3 QDs with excellent crystal quality. The properties of the as-prepared In2S3 QDs were investigated and photodetectors based on the QDs were also fabricated to study the use of the material in optoelectronic applications. The results show that the detectivity of the device stabilizes at ~ 1013 Jones at room temperature under 365 nm ultraviolet light irradiation at reverse bias voltage.
KeywordsIn2S3 QDs Preparation Properties Optoelectronic application
Atomic force microscope
Critical micelle concentration
Fast Fourier transform
Full width at half maximum
High-resolution transmission electron microscope
Sodium dodecyl sulfate
Scanning electron microscope
Transmission electron microscope
X-ray photoelectron spectroscopy
Graphene-like two-dimensional nanomaterials are of great scientific and technological interests [1, 2]. Currently, there has been growing research interests in developing low-dimensional materials that exhibit unique photoelectric properties  and quantum dots (QDs) have gained much attraction . Indium sulfide (In2S3) QDs, which belong to the group III–VI semiconductor materials , have many unique optoelectrical, thermal, and mechanical properties, which are suitable for numerous potential applications. For example, sulfide nanomaterials have experienced rapid development for use in solar cells , photodetectors [7, 8], biological imaging , and photocatalytic degradation . There are various ways of preparing sulfide QDs, and they can be divided into two main categories, namely, ‘top-down’ and ‘bottom-up’ .
However, commonly used bottom-up methods, such as hydrothermal , template[13, 14], and microwave methods , have many limitations that restrict the widespread application of sulfide QDs . To ensure the successful application of sulfide QDs, it is of paramount importance to develop low-cost, facile preparation method that can produce stable, reliable, and high-quality QDs material . In this article, a novel preparation method that allows synthesis of In2S3 QDs at atmospheric temperature conditions has been developed by using indium chloride and sodium sulfide as indium and sulfur source respectively. The physical and photoelectric properties of the as-prepared In2S3 QDs were investigated using multiple characterization techniques.
Photoelectric device based on the In2S3 QDs were fabricated, and results show the detectivity of the device stabilizes at 1013 Jones under 365 nm UV irradiation at room temperature, which demonstrates In2S3 QDs have great potential applications in photodetectors. Compared with other growth methods, the reported approach is mild, facile, environmentally friendly, rapid, and cheap. Therefore, it is suitable for low-cost large-scale production of the device that also yields excellent performances. This work demonstrates a low-cost, effective fabrication technique for future application of sulfide QDs in the field of photoelectric detection.
Sodium sulfide (Na2S·9H2O) was purchased from Tianjin Wind Ship Chemical Testing Technology Co. Ltd., Tianjin China. Indium chloride (InCl3·4H2O) was obtained from Shanghai Aladdin Biochemical Technology Co. Ltd Shanghai, China. Sodium dodecyl sulfate was purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. Dialysis bag (USA spectrum lab’s regenerated cellulose membrane, Mw = 300) was purchased from Shanghai Yibai Economic and Trade Co. Ltd. All of the materials above were purchased commercially and used without further purification.
In2S3 QDs Fabrication
Transmission electron microscope (TEM) images were obtained with a JEM-2100 high-resolution trans-mission microscope operating at 200 kV. The surface morphology and phase image of photovoltaic devices were determined by scanning electron microscope (SEM, FEI Quanta 200) and AFM (atomic force microscope, SPA-400), respectively. XRD analysis was investigated using a Rigaku D/Max-RA X-ray diffractometer with Cu Ka radiation. Raman spectrum was recorded at ambient temperature on a Renishaw in via Raman microscope with an argon-ion laser at an excitation wavelength of 514.5 nm. Optical properties were characterized by UV-vis, UV-vis-NIR (UV-3600), and fluorescence (Hitachi F-7000) spectrometers. Functional groups on the surface of the In2S3 QDs were verified by XPS (X-ray photoelectron spectroscopy) (PHI Versa Probe II) using 72 W, mono Al Ka radiation. J-V and C-V were measured using Keithley 2400 source meter and semiconductor device analyzer (Keysight B1500A), respectively.
Results and Discussion
Structure and Morphology Studies
TEM images of the In2S3 QDs are shown in Fig. 1b–e. It can be seen that In2S3 QDs are evenly distributed and exhibit spheroid morphology. Its particle size distribution follows the Gaussian distribution with size ranging from 1 to 3 nm and FWHM of 1.12 nm. The particle has an average size of 2.02 nm. Figure 1c–e are HRTEM images of the In2S3 QDs showing its lattice fringes for d = 0.271 nm, 0.311 nm, and 0.373 nm, corresponding to the cubic crystal system of 400, 222, and 220 lattice planes respectively . Figure 1i shows a longitudinal profile of the lattice fringes shown in Fig. 1d. The fast Fourier transform (FFT) pattern of the selected region (red dotted square) is shown in Fig. 1d insert, which reveals six bright spots from the 400 plane diffraction, indicating the crystalline structure of the hexagonal system. The scanning electron microscopy (SEM) image of the as-prepared In2S3 QDs is shown in Fig. 1f. As shown, the In2S3 QDs agglomerated to form a relatively compact structure in order to reduce its surface energy. X-ray diffraction (XRD) planes at 400, 222, and 220 of the In2S3 QDs are shown in Fig. 1g and the calculated particle size using the Sheer formula is in good agreement with the measured size from the 400 plane of HRTEM image. Figure 1h shows Raman spectrum of the In2S3 QDs with typical peaks at 304 cm−1 and 930 cm−1 . Atomic force microscopy (AFM) was performed on four randomly selected In2S3 QDs, marked as A, B, C, and D as shown in Fig. 1j, with measured heights of 1.53 nm, 2.35 nm, 1.35 nm, and 2.32 nm (shown in Fig. 1k), respectively. The average height of 1.94 nm from the AFM measurement is very close to that obtained from the TEM.
The band gap energy of the QDs can be estimated from the curve of (αhv)2 vs. photo energy (hv). The estimated Eg of 3.54 eV, as shown in the inset of Fig. 2a, is very close to the calculated value using the Brus equation (Enp=3.50 eV). Photoluminescence (PL) and photoluminescence excitation (PLE)  studies were performed to investigate the optical properties of the In2S3 QDs. It can be seen from Fig. 2b that there is an emission peak at a wavelength between 300 and 450 nm, and the strongest peak intensity is centered at ~ 390 nm under the excitation of Ex = 250 nm. PLE spectra in Fig. 2c show that wavelengths of the characteristic excitation peaks are shorter than the receiving wavelengths (500–540 nm). The broadening of energy gap of In2S3 QDs compared to its bulk material may also be demonstrated by PL and PLE results. The fluorescence of the In2S3 QDs under visible light and 365 nm UV light are shown in Fig. 2c insert. This demonstrates that the In2S3 QDs possess good UV fluorescence properties. X-ray photoelectron spectroscopy (XPS) was also performed to study the chemical bonds of the In2S3 QDs. Figure 2d shows the XPS full scan spectrum, which consists of S2p at 162.5 eV, In3d5/2 at 444.5 eV, and In3d3/2 at 452.5 eV. Besides, there are residual Cl, Na, O, and C from the surfactant and reactant. Core level peaks of S2p and In3d are shown in Fig. 2e, f respectively. The deconvoluted peaks reveal the bonding states of S2p (In-S, C-S), In3d5/2. (In-S, In-O), and In3d3/2 (In-S, In-O).
A novel and facile preparation method to produce high crystal quality In2S3 QDs was developed. The structural, optical, electrical, and photovoltaic properties of the In2S3 QDs have been studied. In the dark field condition, the activation energy (Ea), finger-leading factor (A), built-in potential (Vbi), and depletion layer width (Wd) of the UV photodetector based on In2S3 QDs were obtained. In2S3 QDs were used as the sole photoactive material in the fabricated photodetector that exhibits the highest detectivity (D*) of 2 × 1013 Jones at room temperature under 365 nm UV light illumination without preamplifier. This method is ideal in developing high performance, large array of In2S3 QDs-based UV photoelectric detector at very low cost.
This work was supported by National Natural Science Foundation of China (No. 61106098), Equipment Pre-research Fund under the Equipment Development Department (EDD) of China’s Central Military Commission (CMC) (No.1422030209), and the Innovation Team Program of NORINCO Group (No.2017CX024).
Availability of Data and Materials
The conclusions made in this manuscript are based on the data (main text and figures) presented and shown in this paper.
RL carried out the experiments and drafted the manuscript. LT designed the experiments. LT and QZ supervised the experiments. THL, KST, and SPL participated in the discussion and analyzed the experimental results. LT, THL, KST, and SPL helped to draft and revise the manuscript. YL, YH, and CS helped to characterize the samples. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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